1. Field of the Invention
The present invention relates to light-emitting elements utilizing electroluminescence. Further, the present invention relates to light-emitting devices and electronic devices that include the light-emitting elements and a method of fabricating the light-emitting elements.
2. Description of the Related Art
In recent years, for display units in televisions, cellular telephones, digital cameras, and the like, planar and thin display units have been demanded, and for a display unit that meets this demand, display units that use self-emitting light-emitting elements have been attracting attention. For one kind of self-emitting light-emitting element, there is a light-emitting element that uses electroluminescence; light emission from a light-emitting material can be obtained by application of a voltage to a light-emitting element where a light-emitting material is interposed between a pair of electrodes.
For this kind of self-emitting light-emitting element, pixel visibility is high compared to that of a liquid crystal display, there are advantages in that no backlight is needed and the like, and these self-emitting light-emitting elements are thought to be suitable for use as flat panel display elements. In addition, for this kind of light-emitting element, there is a significant advantage in that these light-emitting elements can be manufactured to be thin and lightweight. Furthermore, response speed being extremely fast is one of the characteristics, as well.
Moreover, because this kind of self-emitting light-emitting element can be formed into a membrane, by formation of an element with a large area, surface emission can easily be achieved. Because this characteristic cannot easily be obtained with point light sources represented by incandescent light bulbs and LEDs or with line light sources such as fluorescent lamps, the utility value for surface light sources, which can be applied to lighting and the like, is high.
Light-emitting elements that use electroluminescence are divided into whether the light-emitting material is an organic compound or an inorganic compound; generally, light-emitting elements using the former are referred to as organic EL elements, and light-emitting elements using the latter are referred to as inorganic EL elements.
When a light-emitting material is an organic compound, electrons and holes are injected into a layer containing a light-emitting organic compound from a pair of electrodes by application of a voltage to a light-emitting element so that a current flows therethrough. The electrons and holes (i.e., carriers) are recombined, and thus the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on this mechanism, this kind of light-emitting element is called a current excitation type light-emitting element.
It is to be noted that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of this kind of light-emitting element, there are a lot of problems which depend on a material, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.